The invention relates to an output driver circuit connected between a first supply terminal and second, lower supply terminal for driving a load and having
an input node for receiving an input signal and an output node for connection to the load; PA1 a pull down switch having a control terminal and a current channel coupled between the output node and the second, lower supply terminal, the control terminal being coupled to the input node and rendering the current channel conductive to discharge the output node in response to a selected signal at the input node; and PA1 a current sink circuit controlling the pull down switch to discharge the output node in response to a selected voltage at the output node, the current sink circuit including (i) a feeder switch having a control input and a current channel coupling the first supply to the control terminal of the pull down switch, and (ii) a first charging path coupling the first supply to the control terminal of the feeder switch in response to receipt of the selected signal at the input node. PA1 the first charging path has a first impedance by which it takes a first time period after receipt of the selected signal at the input node to charge the control terminal of the feeder switch to a threshold which renders the feeder switch conductive, and PA1 the driver circuit further includes an auxiliary charging path coupling the first supply to the control terminal of the feeder switch in response to the selected signal at the input node, the second current path having a lower impedance than the first charging path and charging the control terminal of the feeder switch to the threshold in a time period shorter than the first time period.
Such an output driver circuit is known from U.S. Pat. No. 5,570,044 (Martin et al). One use of the circuit is for interfacing a logic circuit with a load, such as a bus. For example, upon a transition from a logic high to a logic low at the input terminal, the pull down transistor is rendered conductive, rapidly pulling the output terminal from logic high to logic low. The pull down transistor is then turned off, saving power. Under certain load conditions, it is possible for the voltage at the output terminal after having been initially pulled to a logic low to bounce back up to a higher voltage sufficient to be read as a logic high. Thus, an error or glitch is present in that the logic level at the output terminal does not mirror the signal at the input terminal.
To overcome this glitch, the current sink circuit further includes a feeder switch that provides current to charge the control terminal of the pull down switch if the voltage at the output node bounces above a first selected value. The feeder switch is charged by a charging path from the first supply terminal to the base of the feeder switch, and is enabled when the input transitions to a logic low. The response of the circuit in turning on the pull down switch to discharge voltage bounces at the output node is a limiting factor in the speed of the circuit.
Accordingly, it is the object of the invention to improve the speed of the driver circuit in responding to and discharging voltage bounces.
It is another object of the invention to improve the speed without increasing the static power consumption of the circuit.